A magnitude 6.3 earthquake hit northeastern Japan today, the U.S. Geological Survey (USGS) said, but there were no reports of damages or casualties and a tsunami warning was not issued.

The quake, which shook buildings in the capital, Tokyo, struck off the northeast coast of the main island of Honshu at a depth of 20.5 miles (32 km), the USGS said.

Japanese broadcaster NHK and the U.S. Pacific Tsunami Warning Center said there was no danger of a tsunami.

Tokyo Electric Power Co said there were no abnormalities at its nuclear plant in Fukushima, which was severely damaged just over a year ago by a magnitude 9 quake, Japan's strongest on record, which triggered a big tsunami and caused the world's worst nuclear crisis in 25 years.

Nearly 20,000 people were killed in the March 11 quake last year.

On Tuesday, high-speed bullet trains serving northern Japan were halted but resumed later after no damage was reported, NHK said.

There has been a five-fold increase in earthquakes in the Tokyo metropolitan area since the year-ago disaster, the Tokyo University Earthquake Research Institute has said.